This invention relates to a sealing and cooling mechanism for multi-chip modules, and more particularly to a sealing structure for multi-chip modules formed by hermetically sealing high-performance semiconductor devices over a substrate for use in computers and the like.
Along with the continuing increase in the processing speed and storage capacity of computers, semiconductor devices are increasingly faster and more densely integrated. At the same time, however, the increase in heat emitted by semiconductor devices resulting from their growing power consumption is posing a problem, together with the problem of reliability of stable operation of semiconductor devices. Especially in multi-chip modules wherein a large number of semiconductor devices are mounted over a substrate wired in high density, this increased heating poses a major challenge to configure a satisfactory cooling structure.
On the other hand, to maintain the environmental stability of semiconductor devices and their cooling performance as modules for long periods, how to configure a sealing structure which can hermetically enclose semiconductor devices and He gas for enhancing the efficiency of thermal conduction within multi-chip modules is a major technical problem. One known sealing structure for multi-chip modules is disclosed in, for instance, “MCM-D/C Application for High Performance Module,” Proceedings of 1996 International Conference on Multi-chip Modules, pp. 69-74.
This sealing structure is described below with reference to FIG. 5. In FIG. 5, a large number of semiconductor devices 12 are mounted over a wiring board 11 consisting of alumina ceramic. On the bottom side of the wiring board 11 are provided input/output pins 13. A cap board 16 consisting of a highly thermally conductive materials, covering the wiring board 11, is fixed to the circumference of the wiring board 13 with solder 18 to hermetically seal the module. Between the semiconductor devices 12 and the cap board 16 is provided a thermally conductive means, known as thermally conductive compounds 14, each matching one or another of the semiconductor devices 12, to transmit the heat emitted by the semiconductor devices 12 to the cap board. Over the top surface of the cap board 16 is fitted an air-cooled heat sink 17 to radiate heat from the semiconductor devices 12 transmitted via the cap board 16.
The above-described sealing structure for multi-chip modules involves several problems of reliability of the sealing connection and cooling performance. In the above-cited structure described in “MCM-D/C Application for High Performance Module,” Proceedings of 1996 International Conference on Multi-chip Modules, pp. 69-74, highly thermally conductive metals, such as aluminum and copper, are used for the cap and the heat sink to secure satisfactory cooling performance.
On the other hand, a ceramic material, such as alumina ceramic, is used for the wiring board to facilitate fine multi-layered wiring. Because aluminum and copper constitute the cap board or heat sink, and have higher rates of thermal expansion than ceramics, the disparity in thermal expansion between the cap board and heat sink on the one hand and the wiring board on the other hand, widens with an increase in the overall heat emission of the modules and/or in the wiring board size. Because the cap board and the wiring board are fixed with solder in the described structure, deformation in the horizontal direction is constrained with the solder-fixed part as the constraining point as the difference in thermal expansion between the cap board and the wiring board increases.
The quantity of thermal deformation with respect to a module of 150 mm square in size, using aluminum (24×10−6/° C. in thermal expansion coefficient) for the cap and alumina ceramic (7×10−8/° C. in thermal expansion coefficient) for the wiring board is calculated below. Whereas semiconductor devices are usually cooled to keep their temperature at or below 80° C., the temperature of the wiring board then is about 80° C., and that of the cap, around 60° C., though it may vary with the cooling method. Therefore, the relative quantity of thermal deformation against a temperature change from the normal level from the center to a corner of the module can be calculated by the following equation (1):ΔL=LX(α1·ΔT1−1α2·ΔT2)  (1)
ΔL: Relative quantity of thermal deformation
L: Length of member (=150/√2)
α1: Linear expansion coefficient of wiring board (=7×10−6/° C.)
ΔT1: Temperature change of wiring board (=80−20=60° C.)
α2: Linear expansion coefficient of cap (=24×10−6/.C)
ΔT2: Temperature change of cap (=60−20=40° C.)
The relative quantity of thermal deformation predicted by using Equation (1) is 57 μm. If this relative quantity in the horizontal direction is constrained, the module will be bent in the vertical direction by a bimetal effect, resulting in an expanding gap between the cap and the wiring board. Thus, the gap in the thermally conductive compound part applied between the cap and the semiconductor devices will increase, deteriorating the cooling performance, making it more difficult to maintain at a satisfactory level. Moreover, as this thermal deformation gives rise to a large stress in the solder-fixed portion as well, the ceramic and/or the solder part may be destroyed, reducing reliability as well.